For the longest time, many people have been requesting for tutorial on how to paint/speed-paint rocks...

Well, this tutorial shows some of the ways I do it.There are other methods I use but this tutorial shows the best examples to understanding the concept and not just the technique (well, i hope it does).

I hope it helps!

what software is that?

I did this in Photoshop cs2, but anyone can follow these steps using any painting/drawing software. I also just used a default brush here.

Why no textures? Rocks are often rough, right?

What is most important is to depict the form of the rock. To make it look realistic or believable, the rock should have body, and fit into it's environment. The textures are just secondary.